1. Field of Invention
The invention relates to a driving circuit for an active matrix type display using an electro-optical element, such as an organic electroluminescence element (hereinafter referred to as xe2x80x9corganic electroluminescence elementxe2x80x9d), and the like. The invention further relates to a driving method of electronic device and an electronic apparatus, and to the electronic device. More particularly, the present invention relates to a driving circuit having a function for applying reverse bias to an electro-optical element to suppress the deterioration thereof, to a driving method of electronic device and an electronic apparatus, and to the electronic device.
2. Description of Related Art
It is known that a display can be realized by arranging a plurality of pixels in matrix that include an organic electroluminescence element that is one of electro-optical elements. In such a display, the organic electroluminescence element is arranged such that a laminated organic thin film including a light emitting layer is interposed between a cathode formed of a metal electrode, for example, Mg, Ag, Al, Li, and the like and an anode formed of a transparent electrode composed of ITO (indium tin oxide).
FIG. 8 shows an ordinary arrangement of a driving circuit for an active matrix type display using an organic electroluminescence element. In this figure, the organic electroluminescence element is shown as a diode 10. Further, the driving circuit 1 is composed of two transistors Tr1 and Tr2 each composed of a thin film transistor (TFT) and a capacitance element 2 for accumulating electric charge.
Herein both the transistors Tr1 and Tr2 are p-channel type TFTs. The transistor Tr1 can be controlled to be turned on and off according to the electric charge accumulated in the capacitance element 2 in the figure. The capacitance element 2 is charged by a data line VDATA through the transistor Tr2 that is turned on by setting a selection potential VSEL to a low level. When the transistor Tr1 is turned on, a current flows to the organic electroluminescence element 10 through the transistor Tr1. The continuous flow of the current to the organic electroluminescence element 10 permits the element to emit light continuously.
FIG. 9 shows a brief timing chart for the circuit of FIG. 8. As shown in FIG. 9, when data is to be written, the transistor Tr2 is turned on by setting the selection potential VSEL to the low level, whereby the capacitance element 2 is charged. This charge period is a writing period TW in the figure. An actual display period follows the writing period TW. In this period, the transistor Tr1 is turned on by the electric charge accumulated in the capacitance element 2. This period is shown as a display period TH in the figure.
FIG. 10 shows another arrangement of the driving circuit for the organic electroluminescence element. The driving circuit shown in the figure is written in the literature xe2x80x9cThe Impact of Transient Response of Organic Light Organic Light Emitting Diodes on the Design of Active Matrix OLED Displaysxe2x80x9d (1998 IEEE IEDM 98-875). In FIG. 10, reference numeral Tr1 denotes a driving transistor, reference numeral Tr2 denotes a charge controlling transistor, reference numeral Tr3 denotes a first selection transistor, and reference numeral Tr4 denotes a second selection transistor that is turned off during the charge period of a capacitance element 2.
As is well known, the characteristics of transistors are dispersed even if they have the same standard. Accordingly, even if the same voltage is applied to the gates of transistors, a current having a given value does not always flow through the transistors, which may cause irregular luminance and the like. In contrast, in this driving circuit, electric charge is accumulated in the capacitance element 2 based on an amount of current according to a data signal output from a current source 4. Thus, the emitting state of organic electroluminescence can be controlled based on the amount of current according to data.
Herein all the transistors Tr1 to Tr4 are P-channel type MOS transistors. The transistors Tr2 and TR3 are turned on by setting a selection potential VSEL to a low level, which causes electric charge having a value according to the output from the current source 4 to be accumulated in the capacitance element 2. Then, after the selection potential VSEL goes to a high level and the transistors Tr2 and Tr3 are turned off, the transistor Tr1 is turned on by the electric charge accumulated in the capacitance element 2 and the transistor Tr4 is turned on by a data holding control signal Vgp so that a current flows to the organic electroluminescence element 10.
FIG. 11 shows a brief timing chart as to the circuit of FIG. 10, As shown in FIG. 11, when data is to be written by the current source 4, the transistors Tr2 and Tr3 are turned on by setting the selection potential VSEL to the a low level, thereby charging the capacitance element 2. This charging period is a writing period TW in FIG. 11. An actual display period follows the write period TW. During the period in which the data holding control signal Vgp is set to the low level, the transistor Tr1 is turned on, and this turned-on period is a display period TH.
FIG. 12 shows still another arrangement of the driving circuit for the organic electroluminescence element. The driving circuit shown in the figure is the circuit disclosed in Japanese Unexamined Patent Application Publication No. 11-272233. In this figure, the driving circuit includes a transistor Tr1 for supplying a current from a power supply to an organic electroluminescence element 10 when it is turned on, a capacitance element 2 for accumulating electric charge for maintaining the transistor Tr1 in the turned-on state, and a charge controlling transistor Tr5 for controlling the charge of the capacitance element 2 according to an external signal. Note that when the organic electroluminescence element 10 is to emit, a potential Vrscan is maintained to a low level to turn off a charge controlling transistor Tr7. With this operation, no reset signal Vrsig is output. Note that reference numeral Tr6 denotes an adjustment transistor.
The transistor Tr5 is turned on, and the capacitance element 2 is charged by a data line VDATA through a transistor Tr6. Then, the conductance between the source and the drain of the transistor Tr1 is controlled according the charged level of the capacitance element 2, and a current flows to the organic electroluminescence element 10. That is, as shown in FIG. 13, when a potential Vscan is set to a high level to turn on the transistor Tr5, the capacitance element 2 is charged through the transistor Tr6. The conductance between the source and the drain of the transistor Tr1 is controlled according the charged level of the capacitance element 2, and a current flows to the organic electroluminescence element 10. The organic electroluminescence element 10 emits.
Incidentally, it is known that application of reverse bias to an organic electroluminescence element is an effective means to increase the life thereof. This increase of life is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 11-8064.
However, in the method of the publication, additional power supplies such as a negative power source, and the like must be newly prepared to apply reverse bias to the organic electroluminescence element, and the organic electroluminescence element must be controlled so as to permit the reverse bias to be applied thereto.
Accordingly, an object of the present invention is to provide a driving circuit for an active matrix type display capable of applying reverse bias to an electro-optical element such as an organic electroluminescence element, and the like without almost increasing power consumption and cost, to provide a driving method of electronic device and an electronic apparatus, and to provide electronic device.
A first driving circuit for active matrix type display according to the present invention is a driving circuit that drives a display in which a plurality of pixels composed of an electro-optical element are disposed in matrix. The driving circuit includes a first terminal electrically connected to any one of a first power supply line for supplying a first potential and a second power supply line for supplying a second potential lower than the first potential, and a second terminal electrically connected to any one of the first and second power supply lines through the electro-optical element. Further, timing at least exists at which, when the electro-optical element is in a first operating state, the first terminal is electrically connected to the first power supply line and the second terminal is electrically connected to the second power supply line through the electro-optical element, and at which, when the electro-optical element is in a second operating state, the first terminal is electrically connected to the second power supply line and the second terminal is electrically connected to the first power supply line through the electro-optical element.
A second driving circuit for active matrix type display according to the present invention can further include a driving transistor for controlling an operating state of the electro-optical element, a capacitance element for accumulating electric charge for maintaining the driving transistor in a turned-on state, and a charge controlling transistor for controlling the charge to the capacitance element according to an external signal. Further, one of the electrodes constituting the capacitance element is electrically connected to the first terminal and the other electrode constituting the capacitance element is electrically connected to the gate electrode of the driving transistor, and the first terminal is electrically connected to the second terminal through the source and the drain of the driving transistor.
A third driving circuit for active matrix type display according to the present invention can further include a driving transistor for controlling an operating state of the electro-optical element, a capacitance element for accumulating electric charge for maintaining the driving transistor in a turned-on state, and a charge controlling transistor for controlling the charge to the capacitance element according to an external signal. Further, one of the electrodes constituting the capacitance element is electrically connected to the first terminal through a selection transistor that is turned off during the charge period of the capacitance element, the other electrode constituting the capacitance element is electrically connected to the gate electrode of the driving transistor, and the first terminal is electrically connected to the second terminal through the source and the drain of the driving transistor and through the source and the drain of the selection transistor.
A fourth driving circuit for active matrix type display according to the present invention can further include a driving transistor for controlling an operating state of the electro-optical element, a capacitance element for accumulating electric charge for maintaining the driving transistor in a turned-on state; and a charge controlling transistor for controlling the charge to the capacitance element according to an external signal. Further, one of the electrodes constituting the capacitance element is electrically connected to the gate electrode of the driving transistor, the other electrode constituting the capacitance element is electrically connected to the ground, and the first terminal is electrically connected to the second terminal through the source and the drain of the driving transistor.
In short, since a connected state of the first power supply and the second power supply to the driving circuit is changed by switches, reverse bias can be applied to an organic electroluminescence element without almost increasing power consumption and cost. In this case, a first power supply is ordinarily set to Vcc and a second power supply is ordinarily set to the ground (GND), and potentials which are originally prepared are used. However, when a difference of potential that is sufficient for the organic electroluminescence element to emit can be secured, the power supplies are not limited thereto.
In a fifth driving circuit for active matrix type display of the present invention, the electro-optical element can be an organic electroluminescence element.
A first electronic apparatus of the present invention can be an electric apparatus having an active matrix type display that includes the driving circuit.
A first method of driving electronic device of the present invention is a method of driving electronic device including a first power supply line having a first potential, a second power supply line having a second potential that is a potential lower than the first potential, and an electronic device electrically disposed between the first power supply line and the second power supply line. The method can include the steps of electrically connecting one end of the electronic element to the second power supply line when the other end of the electronic element is electrically connected to the first power supply line, and electrically connecting one end of the electronic element to the first power supply line when the other end of the electronic element is electrically connected to the second power supply line.
It should be noted that the terms xe2x80x9celectrically disposedxe2x80x9d are not always limited to the case that an electron element is directly connected to a power supply line and also includes the case that other element such as a transistor or the like is disposed between the power supply line and the electronic element. A liquid crystal element, an electrophoretic element, an electroluminescence element, and the like, for example, are exemplified as the electronic element. Further, the electronic element means a element that is driven when a voltage is applied or a current is supplied thereto.
In a second method of driving electronic equipment of the present invention, the electronic device can be a current-driven device that is driven by a current.
That is, when the electronic device is the current-driven element, a current flows in a forward direction or a reverse direction by the driving method.
A first electronic device of the present invention is an electronic device including a first power supply line having a first potential, a second power supply line having a second potential that is a potential lower than the first potential, and an electronic element electrically disposed between the first power supply line and the second power supply line. The device having one end of the electronic element electrically connected to the second power supply line when the other end of the electronic element is electrically connected to the first power supply line and one end of the electronic element electrically connected to the first power supply line when the other end of the electronic element is electrically connected to the second power supply line.
In second electronic device of the present invention, the electronic element can be disposed in a unit circuit that is disposed in correspondence to the node of a data line for supplying a data signal and a scan line for supplying a scan signal in the above electronic device.
In third electronic device of the present invention, the unit circuit can include a first transistor for controlling the conductivity of the electronic element, a second transistor the gate electrode of which is connected to the scan line, and a capacitance element connected to the gate electrode of the first transistor for accumulating electric charge corresponding to the data signal supplied from the data line.